A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
A lithographic apparatus includes various moving parts that are positioned using at least one positioning device such as a linear or rotating motor or actuator. In a lithographic apparatus, examples of moving parts are a substrate (e.g. wafer) stage, a patterning device (e.g. reticle) stage, a handler (robot arm), etc. A substrate stage may include different positioning devices for moving a wafer support in multiple degrees of freedom to desired positions with a desired speed, acceleration, etc. Likewise, a patterning device stage may include different positioning devices for moving a reticle support in multiple degrees of freedom to desired positions with a desired speed, acceleration, etc.
The process of positioning the substrate support or patterning device support using the positioning device is controlled by a controller. Such a controller may e.g. include a control characteristic being any combination of a proportional (P) control function, an integrating (I) control function, and a differentiating (D) control function.
In order to control the positioning device, the controller receives a position signal from a position sensor detecting the position of the object to be positioned (e.g. the wafer support or the reticle support). The position signal is compared with a setpoint signal defining the desired position of the object. The difference between the position signal and the setpoint signal is obtained as one or more error signals, and on the basis of the one or more error signals, the controller may generate one or more positioning device control signals for controlling the position and movement of the positioning device such as to reduce or eliminate position errors. This method of control is also referred to as feedback control, since the actual position is fed back to a controller input to generate the one or more error signals.
In the process of generating positioning device control signals, the controller shows a control characteristic, which is a feature of the controller. The control characteristic is the way in which the controller operates in response to detecting position errors.
Modeling or designing this control characteristic is often a trade off between disturbance suppression (which can, under linear feedback, e.g. be improved by increasing the integrator control function gain) and sensitivity to noise or a trade off between obtaining a small rising time and a small (or acceptable) overshoot thereby affecting the settle time. As an example, reference can be made to a specific velocity profile (or trajectory) that is encountered during the operation of a lithographic apparatus. In order to perform a scanning exposure process of a wafer or substrate, the following cycle may be performed for each die. In a first procedure, a substrate table, provided with a wafer (or substrate) is accelerated to a predefined velocity. Once this velocity is reached, this velocity needs to be maintained to perform the scanning exposure process. Once the scanning exposure is done, a deceleration phase is, in general, desired. This process is repeated for each die to expose the entire wafer.
Generally, in order to adequately handle time-varying disturbances, in a certain time period, a controller with a specific control characteristic related to the disturbance spectrum in this time period is desired. In another time period this spectrum may change, and therefore a different control characteristic may be desired.
As disclosed in US 2007/0236163, it has been proposed to provide a variable gain controller depending on the type of disturbance experienced. In the controller as described, is it proposed to, for error signals having a magnitude in a predefined range, selectively set the controller gain to a value higher than a value for error signals having a magnitude outside the range.
However, this may result in an amplification of noise which may be present in the error signal.